The present invention relates generally to an analog signal synthesizing processing circuit and, more particularly, to an analog signal synthesizing processing circuit which can periodically perform noise compensation.
Most video tape recorders (hereinafter called "VTR"), such as home-use helical scanning VTRs, have two rotary heads. These heads are mounted on the periphery of a rotary drum and located diametrically opposite to each other. Either head can reproduce signals from a magnetic tape when it is in a sliding contact with the tape.
A signal reproduced by the first rotary head must be combined with that reproduced by the second rotary head, thereby to provide a continuous signal. In order to combine these signals, it is necessary to alternately select the output signals of the two heads. More precisely, the two heads must be alternately switched. This technique is called "head switching". When the head switching is carried out, however, FM carriers will become discontinuous, and noise, generally known as "switching noise", will inevitably be generated. The switching noise is virtually no problem in reproducing image signals since the head switching can be performed during the blanking period. The noise is a great problem in reproducing audio signals since the audio signals have no blanking periods. The quality of the reproduced audio signals is influenced very much by the switching noise.
Accordingly, most high-fidelity VTRs now on sale are provided with an apparatus for removing switching noise. This apparatus replaces the switching noise with signals containing no noise, such as a fixed-level signal generated immediately preceding the noise, a signal having a level or slope estimated from the level or slope of a signal immediately generated preceding the noise, or a signal generated by another reproduction system in which the head switching is performed at different timing. The noise-removing apparatus has a drawback, however. Since it employs analog processing to replace the switching noise with a signal other than the audio signal, the efficiency of removing the switching noise changes in accordance with the frequency of the audio signal.
An apparatus which performs digital processing to remove switching noise has been invented, as is disclosed in Japanese Utility Model Disclosure Sho 61-52879. In this apparatus, a clock signal synchronous with the timing of the head switching is delayed and is used, as a sampling clock signal, thereby sampling the audio signal (i.e., an analog signal). Also in this apparatus, the sampling period is made longer than the noise-generating period, and sampling of the noise is inhibited, thereby to remove the switching noise. Since the sampling period is relatively long, the reproduced analog audio signal will, in some cases, have a waveform greatly different from that of the original audio signal. If the sampling period is shortened in order to reduce this difference, there will be the possibility that the switching noise is adversely sampled and cannot be removed.